Sectioned nitrogen-fixing nodule

Nitrogen-fixing nodules on roots of a tree lupin, Lupinus arboreus, caused by Rhizobium bacteria. The association between Rhizobium species and leguminous plants is an example of symbiosis (mutual benefit). The bacteria enter the plant roots and induce the formation of tumour-like nodules. Within the nodule, the bacteria proliferate and develop the ability to fix atmospheric nitrogen. This fixed nitrogen is beneficial to the plant for its growth, and the bacterium benefits from carbon compounds made by the plant. The pictures shows large nodules growing on a secondary root. In the centre, the nodule has been sliced open. Nitrogen fixation involves the use of leghaemoglobin, an iron-containing protein analogous to haemoglobin. Areas of pink within the nodule show locations at which active nitrogen fixation is occurring. The sliced nodule is approximately 2.5cm by 1cm in size